1. Field of the Invention
The present invention relates to systems and methods of surface water purification.
2. Description of Related Art
The removal of pollutants from surface waters, such as lakes, ponds, streams, and canals, has proven technically challenging and costly. Chemical coagulants have been used successfully to treat surface waters, but these chemical-based treatment systems can be expensive and problematic with respect to environmental concerns about the disposal of chemical residues.
Surface water chemical treatment systems usually obtain feedwaters from a polluted source via either a gravity flow or pumped flow of water through a pipe or culvert. A chemical coagulant is injected into the pipe, and water turbulence provides mixing of the coagulant and water. This coagulant-water mixture is then fed into a large settling basin, typically an earthen pond, with a relatively long (e.g., 3-24 hour) hydraulic retention time. A chemical precipitate (termed a “floc”) forms in the water as a result of coagulant addition. The flocs and associated pollutants settle in the quiescent pond environment, and the clarified water, relatively free of pollutants, exits the pond.
Over time, a layer of floc or chemical residue builds up on the bottom of the settling pond, and this material periodically must be collected, dried, and disposed of. A typical approach used for chemical residue handling is to establish and maintain chemical floc “drying beds” adjacent to the pond, into which the floc material periodically is pumped from the settling basin and allowed to dry. The dried chemical residue and associated pollutants typically is then hauled away to a disposal site. The selection of an appropriate disposal site, however, can be a problem, due to concerns about adverse environmental effects of the residue constituents (which include the active ingredient of the coagulant, often a metal, and the pollutants that have been removed from the water).
Chemical treatment systems require moderate amounts of land, with the settling pond and the floc residue drying beds comprising the greatest area requirement. There exists a compromise between settling pond size and chemical dose; to a certain extent, the larger the settling basin (and the more effective it is for floc removal), the lower the chemical dose required to accomplish effective pollutant removal. Because operating costs associated with chemical use can be quite high, it is important to incorporate adequate settling basin area in the treatment system.